The present invention relates to apparatus for determining the charge state of batteries, particularly nickel cadmium or nickel metal hydride rechargeable batteries.
Batteries, such as alkaline, nickel cadmium, nickel metal hydride, and lead acid, are used in a wide range of devices from flashlights to portable computers. In applications such as powering portable computers, the user is keenly interested in knowing how much operating time remains and when the battery must be either recharged or replaced. This information can be derived from the charge state of the battery.
Batteries are complex electrochemical devices which have characteristics that make the accurate determination of their charge state difficult. Charge and discharge characteristics of batteries are not linear with respect to battery voltage, temperature, and current. Batteries have an internal series resistance which results in a voltage drop under load. The battery voltage also varies with temperature, although this temperature variation is more noticeable with certain battery types than with others. These characteristics of the battery may vary with the charge state and over the life of the battery.
Since the voltage of a battery drops as the battery discharges, one method of measuring charge state is to measure the battery voltage and estimate the charge state from that measured voltage. Such a measurement will be inaccurate due to the voltage drop caused by the internal series resistance of the battery if the measurement is made under load. For some battery types, especially nickel-cadmium batteries, the battery voltage change with temperature, and the nonlinearity of battery voltage over time also makes simple measurements inaccurate.
What is needed is a method for accurately determining the charge state of a battery that takes into account the nonlinearity of battery voltage over time, compensates for battery voltage change due to temperature, and compensates for the internal resistance of the battery.